1. Technical Field
The present invention relates to so-called scanning rangefinders, designed to informationally receive, with an electromagnetic wave receiver connected to a distance computation circuit, reflection of electromagnetic waves that from an electromagnetic wave projector have been directed onto and scanned over scanning targets, and to computationally gauge the distance to the scanning targets.
2. Description of the Related Art
To date, two configurations, as represented in FIGS. 9 and 10, have been known for scanning rangefinders that use a rotating mirror(s) and wobble its optical axis over an entire 360-degree circuit. In either configuration the optical axis of the mirror(s) and the shaft of the motor that rotates the mirror(s) are made coincident.
The rangefinder in FIG. 9 is structured to employ a double-shaft motor 42 from which common rotary shafts 41a and 41b are jutted out vertically, with a scanning mirror 43 on the shaft 41a on the one hand, and a receiving mirror 44 on the shaft 41b on the other, being mounted in phase with each other. (At 45 in FIG. 9 is a beam projector; at 46, a ray receiver; at 47, a projection lens; and at 48, a receiving lens.) This configuration enables the rangefinder sensitivity to be raised, in that because the scanning optical system and the receiving optical system are completely separated, there is little straying of rays from the projection optical system into the receiving optical system, and in that there is little concern that surface reflection from the inner side of a scanning/receiving transparent window 53, nor that rays reflected from debris clinging to the transparent window 53, will enter the ray receiver 46.
The rangefinder in FIG. 10 is structured to employ a motor 42 from which a rotary shaft 41c is jutted upward, with a dual scanning/receiving mirror 49 being mounted on the rotary shaft 41c. Rays output from the beam projector 45 pass through a projection lens 50, is reflected downward by a semitransparent mirror 51, and is shone onto the dual scanning/receiving mirror 49; the rays reflected there are deflected leftward by the mirror 49 and cast onto a subject to be illuminated. Reflected rays from the scanned object are deflected upward by the scanning/receiving mirror 49, are transmitted through the semitransparent mirror 51, pass through a receiving lens 52, and enter the ray receiver 46. With this configuration there is no blind spot even at short range, since the scanning mirror (semitransparent mirror 51) and the receiving mirror (dual scanning/receiving mirror 49) are arranged coaxially on the motor 42; and there is a high degree of flexibility in installing the rangefinder, because the scanning mirror and receiving mirror are disposed unilaterally with respect to the motor 42.
Nevertheless, scanning rangefinders of the FIG. 9 structure suffer from the following drawbacks.    1) The fact that the motor 42 is disposed between the scanning mirror 43 and the receiving mirror 44 makes the distance between the optical axes of the scanning beam and received rays necessarily large. Consequently, at short range reflected rays do not enter the ray receiver 46, which produces a blind spot.    2) Because the center of the optical system is the center of the rangefinder, and the rangefinder is vertically extensive, restrictions on how the rangefinder may be installed result.
In turn, scanning rangefinders of the FIG. 10 structure suffer from the following disadvantages.    1) The semitransparent mirror 51 is used to make the optical axes of the scanning beam and received rays coincide, but in dividing rays the semitransparent mirror 51 lowers the amount of radiation by approximately one-half. This drop in luminous energy means that the power of the laser radiation from the beam projector 45, and the amplifying characteristics of the ray receiver 46, must be jacked up by four times overall, compared with the spilt-optics type of rangefinder of FIG. 9.    2) Inasmuch as the dual scanning/receiving mirror 49 is employed, projection-beam surface reflection off the inner side of the scanning/receiving transparent window 53, and rays reflected from debris clinging to the transparent window 53, enters the ray receiver 46 by way of the scanning/receiving mirror 49 and the semitransparent mirror 51, becoming noise, which consequently is prohibitive of heightening the radiation-receiving sensitivity of the rangefinder.